JPH0751777B2 - Cast-in-place - Google Patents

Description

The present invention relates to cast-in-place piles, and more particularly to reduction of the number of piles.

[Prior Art] Pile foundations are used to ensure sufficient ground strength against loads such as buildings and civil engineering structures. The piles can be roughly divided into ready-made piles and cast-in-place piles.

Ready-made piles are made on site or in factories and consist of concrete piles or steel piles. As for the cast-in-place pile, there are an excavation method of placing concrete after excavation and a penetration method of placing concrete after penetration. This cast-in-place pile has less noise and vibration than the hammering method for ready-made piles, and since it does not drive in, it has little effect on neighboring structures due to vibration and is widely used for construction in urban areas. As described above, the piles are classified according to the manufacturing method and the installation method, but all the piles are linear piles.

[Problems to be Solved by the Invention] Usually, the supporting force of a pile is considered to be the sum of the supporting force at the tip of the pile and the supporting force due to friction of the peripheral surface of the pile. The pile tip supporting force is proportional to the area of the pile tip, and the supporting force due to friction on the peripheral surface is proportional to the pile perimeter. That is, when a straight pile is used as in the conventional case, the bearing capacity of the pile is proportional to the outer diameter of the pile. The cast-in-place pile can have a larger outer diameter than the ready-made pile, but there is a limit to increasing the outer diameter. Therefore, there is a disadvantage in that a large number of piles must be used in order to ensure sufficient ground resistance against the load of the structure or the like.

In the conventional cast-in-place concrete pile, for example, assuming that the normal allowable compressive strength of concrete is 60 kg / cm ² and the N value of the support layer is 50, the ground bearing capacity of the tip is 25 kg / cm ² , and the concrete strength is fully utilized. In order to do so, it is often the case that the bottom of the pile is expanded. By expanding the bottom of the pile tip in this way, the vertical load constantly supported by the pile increases. Since the horizontal force on the pile head used for seismic design is almost always represented by the product of vertical load and horizontal seismic intensity, the horizontal force also increases as the vertical load increases. In order for the pile head to withstand the bending moment due to this horizontal force, the pile head may have to be expanded in diameter, which increases the construction cost.

The present invention has been made to solve such disadvantages, and an object of the present invention is to obtain a cast-in-place pile that can increase the supporting force without expanding the bottom of the pile.

[Means for Solving the Problem] The cast-in-place pile according to the present invention is characterized in that it is formed in an arc shape having a radius of a length substantially the same as the vertical length up to the tip of the pile.

[Operation] In the present invention, by making the pile arc-shaped, the area of the portion of the support layer that receives the ground reaction force in the vertical direction is increased to increase the ground reaction force, and at the same time, increase the length of the entire pile. To increase the frictional force on the peripheral surface.

[Embodiment] FIG. 1 is a longitudinal sectional view showing an embodiment of the present invention. As shown in the figure, the ground with the support layer 2 with N = 50 under the silt layer 1 with N = 10 and depth L ₁ (m) has a shaft diameter D (mm) and a vertical length to the tip. Radius R (m) of almost the same length as length L
The case where an arc-shaped cast-in-place pile (hereinafter referred to as an arc pile) 3 is embedded in the support layer 2 by L ₂ (m) is shown.

Hereinafter, the advantages of using the arc pile 3 will be described in comparison with the case of the straight pile.

The ultimate bearing capacity R _U (tf) of the circular arc pile 3 is determined by the sum of the pile tip resistance, the peripheral frictional force, and the upward ground reaction force, and is expressed by the following equation (1).

R _U = q _d · A + UΣL _i · f _i + Q _z (1) where A: pile tip area (m ² ) q _d : ultimate bearing capacity of pile tip (tf / m ² ) U: pile circumference (M) L _i : Length of pile on which peripheral frictional force acts (m) f _i : Maximum peripheral frictional force of a layer on which peripheral frictional force acts (tf
/ m ² ) Q _z : Upward ground reaction force (tf) For example, the depth of the silt layer 1 L ₁ = 27 (m). Rooting depth L ₂
= 3 (m), shaft diameter D = 609.6 (mm), thickness 9.5 (mm)
Pile tip resistance q _d when the inside of the steel tube 4 concrete 5 was filled, was constructed an arc pile 3 of arc having a radius R = 30 (m)
・ A is q _d · A = 300 × (π / 4) × 0.6096 ² = 87.51 (tf), where the estimated value of q _d in the support layer 2 is 300.

The length L _1R of the circular arc pile 3 in the silt layer 1 is 33.6.
(M), the length L _2R of the supporting layer 2 is 13.5 (m), so the peripheral frictional force is UΣL _i · f _i = 0.609π {(33.6 × 0.1 × 10) + (13.5 × 0.1 × 50 )} = 193.5 (tf).

Further, the upward ground reaction force Q _z acts on the entire arc pile 3, but the supporting layer 2 having a large vertical component force and an N value of 50 or more is used.
Considering only the part embedded in the ground, and considering the ground reaction force to be small, upward ground reaction force in the support layer 2
Q _z2 can be expressed by the following equation.

Q _z2 = L _2R · K _U · δ · D (2) However, L _2R : Length to receive vertical ground reaction force (cm) K _U : Ground reaction force coefficient (Kg / cm ³ ) δ: Settlement Amount (cm) D: Pile outside diameter (cm) where ground reaction force coefficient K _U = 1 (Kg / cm ³ ), subsidence amount δ = 1
(Cm), Q _z2 = 82.2 (tf).

Therefore, the ultimate bearing capacity R _U of the circular pile 3 becomes the sum of these, and R _U = 363.2 (tf).

On the other hand, as shown in Fig. 2, considering the case where the straight pile 6 is constructed on the same ground, the ultimate bearing capacity R _U of the straight pile 6 is R _U = q _d · A + UΣL _i · f _i = 167.9 (tf ) become.

That is, the ultimate bearing capacity R _U of the circular pile 3 is about twice that of the straight pile 6. Therefore, in order to obtain the same ultimate bearing capacity R _U , the number of piles can be reduced to half by using the circular arc piles 3.

In addition, comparing the allowable pull-out force P _U of the circular pile 3 and the straight pile 6, the allowable pull-out force P _U is determined by the peripheral frictional force UΣL _i · f _i ,
In the case of straight pile 4, it is 80.39 (tf / piece), whereas in the case of circular pile 3, it is 193.5 (tf / piece), and the allowable pull-out force P _{U is}
In consideration of the above, the number of circular arc piles 3 can be reduced to less than half that of the straight piles 6.

In the case of the circular arc pile 3, the ground further resists the pulling-out of the pile, and thus the pulling-out force can be increased.

Considering the buckling of the circular arc pile 3, the buckling load N _cr is expressed by the following equation.

N _cr = m _c E1 _y / (Rθ) ² (3) where m _c is a value determined by the support conditions of the pile E: Young's modulus of the pile material I _y : Moment of inertia of the pile Rθ: Buckling length Here, m _c = 32, E = 2.1 × 10 ⁶ (Kg / cm ² ), I _y = π (D ⁴ −
D _in ⁴ ) / 64 (D _in : Steel pipe 4 inner diameter of arc pile 3), Buckling length R
When the length from the pile head to the pile bottom is taken as θ, the buckling load of the circular arc pile 3 becomes N _cr = 244 (tf).

On the other hand, considering the allowable bearing capacity R _a safety factor of the arc pile 3 3 consists ultimate bearing capacity R _U in _{R a = R U / 3 =} 121 (tf), the buckling load N _cr = 244 (tf It is about 1/2 of that of () and there is no risk of out-of-plane buckling. In the actual case, the circular arc pile 3
The buckling load N _cr is much larger than that obtained by the above model because all directions are constrained.

Next, an example of a construction method of the circular arc pile 3 will be described.

For the construction of the circular arc pile 3, for example, as shown in FIG. 3, a tip drive type drill attached to the tip of the inner pipe 11 curved in the longitudinal direction.
Use 12

The tip drive type drill 12 comprises a guide portion 13 and a rotating portion 14 attached to the tip portion of the guide portion 13. The rotating portion 14 has a pilot bit 15 at the tip and is raised at a plurality of positions on the circumferential side surface. It has a retractable retract bit 16.

Then, while guiding the guide portion 13 of the tip drive type drill 12 on the inner surface of the steel pipe 4 which is curved in an arc shape having the radius R of the arc pile 3, the rotating portion 14 is projected from the tip of the steel pipe 4, and the pilot bit 15 and the retract. The inner pipe 11 and the steel pipe 4 are propelled while excavating the underground with the bit 16. Excavation water used for this excavation is supplied to the front surface of the pilot bit 15 through the inner pipe 11, and is discharged through the space between the inner pipe 11 and the steel pipe 4, which contains the waste sludge.

After excavating the ground in this way and installing the steel pipe 4 at a predetermined position, the inner pipe 11 and the tip drive type drill 12 are pulled out, concrete is placed in the steel pipe 4, and the arc pile 3 is completed.

In addition, in the said Example, although the case where the circular arc pile 3 was formed with the steel pipe concrete pile was demonstrated, even if the circular arc pile 3 is formed with a concrete pile or a reinforced concrete pile, the same effect as the said Example is produced. it can.

[Effect of the Invention] As described above, the present invention increases the ground reaction force by increasing the area of the portion of the support layer that receives the ground reaction force in the vertical direction by increasing the ground reaction force by forming the pile in an arc shape. Since the whole length is increased to increase the frictional force on the peripheral surface and increase the supporting force, the number of piles to be installed can be reduced.

Further, by reducing the number of piles, the construction period can be shortened and the construction cost can be reduced.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention, (a) is a vertical sectional view of an arc pile, (b) is a distribution diagram of N values, and FIG. 2 (a) is a vertical sectional view of a straight pile, (b). Is a distribution chart of N values, and FIG. 3 is an explanatory view showing a state when installing an arc pile. 1 ... silt layer, 2 ... support layer, 3 ... arc pile, 4 ...
Steel pipe, 5 ... Concrete.

Claims (1)

[Claims] 1. A cast-in-place pile, which is formed in an arc shape having a radius substantially the same as the vertical length to the tip of the pile.